Effect of cloning rate on fitness-related traits in two marine hydroids

Hydractinia symbiolongicarpus and Podocoryna carnea are colonial marine hydroids capable of reproducing both sexually and asexually. Asexual reproduction, by colony fragmentation, produces a genetic clone of the parent colony. This study examines the effect of very different cloning rates on colony growth rate, oxygen uptake rate, and colony morphology. Colonies of one clone of each species were maintained for an extended time in two treatments: in a state of constant vegetative growth by repeated cloning, and in a state restricted from vegetative growth (no cloning). For both species, tissue explants taken from the growing colonies grew more slowly than similar explants taken from the restricted colonies. For one species, tissue explants from the growing colonies used oxygen at a higher rate than similar explants from restricted colonies; for the other species, no difference was detected, although the sample size was small. For both species, tissue explants from restricted colonies formed more circular, "sheet-like" shapes, whereas those from their growing counterparts formed more irregular, "runner-like" shapes. After these experiments, in the third winter of treatment, all colonies experienced a severe tissue regression. Within 6 months after this event, the colonies had regrown to their former sizes. A growth assay at this point revealed no difference in growth rate, possibly suggesting an epigenetic basis for these results. Changes in clonal growth rates and morphology correlated with variation in fragmentation rate might affect the ecology of these and other clonal organisms.     

Reactive oxygen species and the regulation of hyperproliferation in a colonial hydroid

Colonies of Podocoryna carnea circulate gastrovascular fluid among polyps via tubelike stolons. At polyp-stolon junctions, mitochondrion-rich cells in part regulate this gastrovascular flow. During competition, colonies hyperproliferate nematocytes and stolons; nematocysts are discharged until one colony is killed. Hyperproliferation then ceases, and normal growth resumes. Here, competing colonies were treated with azide, which inhibits respiration and upregulates reactive oxygen species (ROS). After the cessation of competition, azide-treated colonies continued to hyperproliferate. In azide-treated competing colonies, however, mitochondrion-rich cells were found to produce similar amounts of ROS as those in untreated competing colonies. Subsequent experiments showed that both azide treatment and competition diminished the lumen widths at polyp-stolon junctions, where mitochondrion-rich cells are found. In competing colonies, these diminished widths may also diminish the metabolic demand on these cells, causing mitochondria to enter the resting state and emit more ROS. Indeed, results with two fluorescent probes show that mitochondrion-rich cells in competing colonies produce more ROS than those in noncompeting colonies. In sum, these results suggest that competition perturbs the usual activity of mitochondrion-rich cells, altering their redox state and increasing ROS formation. Via uncharacterized pathways, these ROS may contribute to hyperproliferation.     

Extreme plasticity in thermoregulatory behaviors of free-ranging black-tailed prairie dogs

In the natural environment, hibernating sciurids generally remain dormant during winter and enter numerous deep torpor bouts from the time of first immergence in fall until emergence in spring. In contrast, black-tailed prairie dogs (Cynomys ludovicianus) remain active throughout winter but periodically enter short and shallow bouts of torpor. While investigating body temperature (T(b)) patterns of black-tailed prairie dogs from six separate colonies in northern Colorado, we observed one population that displayed torpor patterns resembling those commonly seen in hibernators. Five individuals in this population experienced multiple torpor bouts in immediate succession that increased in length and depth as winter progressed, whereas 16 prairie dogs in five neighboring colonies remained euthermic for the majority of winter and entered shallow bouts of torpor infrequently. Our results suggest that these differences in torpor patterns did not result from differences in the physiological indicators that we measured because the prairie dogs monitored had similar body masses and concentrations of stored lipids across seasons. Likewise, our results did not support the idea that differences in overwinter T(b) patterns between prairie dogs in colonies with differing torpor patterns resulted from genetic differences between populations; genetic analyses of prairie dog colonies revealed high genetic similarity between the populations and implied that individuals regularly disperse between colonies. Local environmental conditions probably played a role in the unusual T(b) patterns experienced by prairie dogs in the colony where hibernation-like patterns were observed; this population received significantly less rainfall than neighboring colonies during the summer growing seasons before, during, and after the year of the winter in which they hibernated. Our study provides a rare example of extreme plasticity in thermoregulatory behaviors of free-ranging prairie dogs and provides evidence contrary to models that propose a clear delineation between homeothermy, facultative torpor, and hibernation.     

Competition as a selective mechanism for larger offspring size in guppies

Highly competitive environments are predicted to select for larger offspring. Guppies Poecilia reticulata from low-predation populations have evolved to make fewer, larger offspring than their counterparts from high-predation populations. As predation co-varies with the strength of competition in natural guppy populations, here I present two laboratory experiments that evaluate the role of competition in selecting for larger offspring size. In the first experiment, paired groups of large and small newborns from either a high- or a low-predation population were reared in mesocosms under a high- or a low-competition treatment. While large newborns retained their size advantage over small newborns in both treatments, newborn size increased growth only in the high-competition treatment. Moreover, the increase in growth with size was greater in guppies derived from the low-predation population. In the second experiment, pairs of large and small newborns were reared in a highly competitive environment until reproductive maturity. Small size at birth delayed maturation and the effect of birth size on male age of maturity was greater in the low-predation population. These results support the importance of competition as a selective mechanism in offspring size evolution.     

A discrete stage-structured two-species competition model with sexual and clonal reproduction

In this paper, we analyse a discrete stage-structured model which is a generalization of the two-species competition model studied in [2]. Motivated by plant populations, each species is assumed to reproduce both sexually and clonally. We show that this model has a dynamical behaviour that is similar to that of the classical continuous two-dimensional Lotka-Volterra model under weak nonlinearities of the Beverton-Holt type. By allowing the species to have different competition efficiencies, we show that it is possible to obtain different dynamics including coexistence, bistability and competitive exclusion, in contrast with the model studied in [2], which exhibits only competitive exclusion behaviour.     

Gamete production patterns,ploidy, and population genetics reveal evolutionary significant units in hybrid water frogs (Pelophylax esculentus)

The European water frog Pelophylax esculentus is a natural hybrid between P. lessonae (genotype LL) and P. ridibundus (RR). It reproduces through hybridogenesis, eliminating one parental genome from its germline and producing gametes containing the genome of the other parental species. According to previous studies, this elimination and transmission pattern is very diverse. In mixed populations, where only diploid hybrids (LR) live in sympatry and mate with one or both parental species, the excluded genome varies among regions, and the remaining genome is transmitted clonally to haploid gametes. In all‐hybrid populations consisting of diploid (LR) and triploid (LLR and/or LRR) frogs, diploid individuals also produce gametes clonally (1n in males, 2n in females), whereas triploids eliminate the genome they have in single copy and produce haploid gametes containing the recombined other genome. However, here, too, regional differences seem to exist, and some triploids have been reported to produce diploid gametes. In order to systematically study such regional and genotype differences in gamete production, their potential origin, and their consequences for the breeding system, we sampled frogs from five populations in three European countries, performed crossing experiments, and investigated the genetic variation through microsatellite analysis. For four populations, one in Poland, two in Germany, and one in Slovakia, our results confirmed the elimination and transmission pattern described above. In one Slovakian population, however, we found a totally different pattern. Here, triploid males (LLR) produce sperm with a clonally transmitted diploid LL genome, rather than a haploid recombined L genome, and LR females clonally produce haploid R eggs, rather than diploid LR eggs. These differences among the populations in gamete production go along with differences in genomotype composition, breeding system (i.e., the way triploids are produced), and genetic variation. These differences are strong evidence for a polyphyletic origin of triploids. Moreover, our findings shed light on the evolutionary potential inherent to the P. esculentus complex, where rare events due to untypical gametogenetic processes can lead to the raise, the perpetuation, and the dispersion of new evolutionary significant lineages which may also deserve special conservation measures.     

The interplay between density- and trait-mediated effects in predator-prey interactions: a case study in aphid wing polymorphism

Natural enemies not only influence prey density but they can also cause the modification of traits in their victims. While such non-lethal effects can be very important for the dynamic and structure of prey populations, little is known about their interaction with the density-mediated effects of natural enemies. We investigated the relationship between predation rate, prey density and trait modification in two aphid-aphid predator interactions. Pea aphids (Acyrthosiphon pisum, Harris) have been shown to produce winged dispersal morphs in response to the presence of ladybirds or parasitoid natural enemies. This trait modification influences the ability of aphids to disperse and to colonise new habitats, and hence has a bearing on the population dynamics of the prey. In two experiments we examined wing induction in pea aphids as a function of the rate of predation when hoverfly larvae (Episyrphus balteatus) and lacewing larvae (Chrysoperla carnea) were allowed to forage in pea aphid colonies. Both hoverfly and lacewing larvae caused a significant increase in the percentage of winged morphs among offspring compared to control treatments, emphasising that wing induction in the presence of natural enemies is a general response in pea aphids. The percentage of winged offspring was, however, dependent on the rate of predation, with a small effect of predation on aphid wing induction at very high and very low predation rates, and a strong response of aphids at medium predation rates. Aphid wing induction was influenced by the interplay between predation rate and the resultant prey density. Our results suggests that density-mediated and trait-mediated effects of natural enemies are closely connected to each other and jointly determine the effect of natural enemies on prey population dynamics.     

Potential for intraguild predation and competition among predatory Coccinellidae and Chrysopidae

In a laboratory study two coccinellid species, Coleomegilla maculata (DeGeer) and Harmonia axyridis Pallas, completed preimaginal development on lacewing eggs, Chrysoperla carnea Stephens or pea aphids, Acyrthosiphon pisum (Harris) in similar times. Preimaginal survival on C. carnea eggs was similar to survival on A. pisum for all stages of C. maculata and H. axyridis. Coccinellid adults that developed on C. carnea eggs were smaller than adults reared on A. pisum. Coccinella septempunctata L. did not complete preimaginal development on C. carnea eggs. Chrysoperla carnea preimaginal developmental time was approximately 20 days when fed either C. maculata eggs or A. pisum. Chrysoperla carnea fed C. maculata eggs developed into smaller adults, compared to adults reared as larvae on A. pisum, Ostrinia nubilalis (Hübner) eggs, or A. pisum alternated daily with O. nubilalis eggs. C. carnea did not complete preimaginal development on H. axyridis eggs. Cannibalism occurred more frequently between C. carnea third instars than between C. maculata fourth instars. When a C. carnea third instar was paired with a C. maculata fourth instar, more C. maculata were preyed upon by C. carnea, regardless of the herbivorous prey density. In the field these two predator species may negatively affect each other and reduce their suppression of pest densities.     

Interaction between founder effect and selection during biological invasion in an aquatic plant

Long-distance colonization and rapid range expansion associated with biological invasion may have major evolutionary consequences via both stochastic processes and selection. Using large-scale population genetic surveys, we demonstrate a major shift in the relative frequency of sexually fertile diploid versus sexually sterile triploid populations associated with the invasion of North America by a clonal aquatic plant, Butomus umbellatus. Most populations across the native European range were triploid (84% of 108), whereas most introduced populations were diploid (71% of 136). We evaluated the roles of stochastic processes versus natural selection in causing this shift by surveying predominantly neutral genetic variation at 28 RAPD loci. In Europe (EU) we detected 47 distinct genotypes among 142 plants sampled from 71 populations, whereas in North America (NA) we detected only six genotypes among 138 plants from 69 populations. Of the six NA genotypes, a set of four closely related genotypes were found only in triploid populations and a pair of closely related genotypes were found only in diploid populations, and these were genetically divergent from the triploid genotypes. This result is consistent with severe founder effect. Because sex creates genotypic variation and produces offspring with greater dispersal potential than those produced clonally, we tested the hypothesis that sexual reproduction characteristic of diploids has given them a colonization advantage that accounts for their high frequency in NA. However, we found little or no evidence of sexual recruitment in introduced diploids. One very widespread heterozygous genotype occurred in 95% of 38 introduced diploid populations (i.e., 72 of 76 plants surveyed) suggesting predominant clonal reproduction. Moreover genotypic diversity was not higher within or among diploid than triploid populations in either the native or introduced range. Low genetic diversity in diploid populations was also supported by a comparison of within-population quantitative variation for plant size under a common greenhouse environment. Thus, diploids have not been favored during colonization owing to their sexual fertility. However, concurrent studies have shown that NA diploids exhibit a much higher capacity for clonal reproduction, via small vegetative bulbils, than NA triploids, which almost never produce bulbils. The same difference in clonal capacity is not a consistent feature of the native EU populations. Taken together, these results suggest that strong founder effect has set the stage for a major increase in diploid frequency due to the particular, and possibly idiosyncratic, features of the diploid and triploid lineages introduced to North America.     

Genetic diversity and the genetic structure of natural populations of Chamaecyparis obtusa: implications for management and conservation

We investigated 25 natural populations of Chamaecyparis obtusa using 51 cleaved amplified polymorphic sequence (CAPS) markers, which were developed using information on sequence-tagged sites (STS) in Cryptomeria japonica. Most CAPS markers have codominant expression patterns, and are suitable for population studies because of their robustness and convenience. We estimated various genetic diversity parameters, including average heterozygosity (H(e)) and allelic richness and found that the more peripheral populations tended to have lower genetic diversity than central populations, in agreement with a previous theoretical study. The overall genetic differentiation between populations was low, but statistically significant (G(ST)=0.039), and similar to the level reported in a previous allozyme study. We attempted to detect non-neutral loci associated with local adaptation to clarify the relationship between the fixation index (F(ST)) and H(e) values for each locus and found seven candidates non-neutral loci. Phylogenetic tree analysis of the populations and Bayesian clustering analysis revealed a pattern of gradually increasing isolation of populations with increasing geographical distance. Three populations had a high degree of linkage disequilibrium, which we attribute to severe bottlenecks due to human disturbance or competition with other species during their migration from refugia after the most recent glaciation. We concluded that the small populations in western Japan and in Kanto district are more important, from a conservation perspective, than the populations in central Japan, due to their genetic divergence, relatively small sizes and restricted areas.     

Small and rough colony pseudomonas aeruginosa with elevated biofilm formation ability isolated in hospitalized patients

Pseudomonas aeruginosa is a key pathogen of nosocomial infection, and causes persistent infection in patients with specific diseases like cystic fibrosis (CF). It has been reported that patients affected with CF discharge, at a high frequency, small colony variants with high adherence ability. In routine laboratory testing, we found atypical small and rough type (SR) colony variants of P. aeruginosa. The SRs and the counterpart wild type (WT) colonies showed similar biochemical features, antimicrobial susceptibilities, pulsed-field gel electrophoresis (PFGE) profiles, serotypes, and twitching motilities. The biofilm formation abilities of all the SR colonies, however, were extremely elevated as compared to those of the counterpart WT colonies. The frequency of SR-positive patients was 3.1% of the P. aeruginosa-positive inpatients (5/160), and that of the SR isolates was 0.6% of the P. aeruginosa strains (6/970) isolated in our laboratory over a period of 6 months. The SR-positive patients did not have any common disease or particular antibiotics treatment. The PFGE profiles showed that the SRs and the counterpart WTs were identical to each other, and also that three of the five SR/WT pairs were clonally similar. The three pairs were recovered from the feces, urine, and endotracheal secretion, respectively, of three patients hospitalized in two distinct wards. The results suggest that P. aeruginosa spontaneously produced highly adherent SR colonies in hospitalized patients, and these colonies may tend to spread in a hospital.     

Seed weight and seed number affect subsequent fitness in outcrossing and selfing Primula species

Using the outcrossing Primula farinosa and its autogamous selfing relatives P. scotica , P. scandinavica and P. stricta , we compared the fitness of light and heavy seeds. Heavy seeds germinated in greater numbers and more quickly. In competition with seedlings grown from lighter seeds, heavy seeds produced larger rosettes. In P. farinosa such seedlings went on to produce more seeds, and in two populations heavier seeds, than plants from lighter seeds. After transplantation to natural populations, seedlings of P. farinosa derived from heavy seeds produced larger rosettes, more flowers and seeds than those from lighter seeds in certain populations so that seedlings born of heavy seeds were much fitter than seedlings from lighter seeds. Average seed weight varied in inverse proportion to seed number per capsule. The autogamous species produced on average about twice as many seeds per capsule as P. farinosa . In P. scotica and P. stricta this difference appears to be due in part to assured fertilization, but this high fecundity did not cause disadvantageously light seeds. As these species produced fewer capsules per scape, their overall seed production was on average no greater than for P. farinosa . P. farinosa traded-off fitness between capsules with large seed numbers, which donated more offspring to the next generation, and those with small seed numbers, whose heavy seeds would be more likely to reproduce themselves in the next generation. We conclude that low fecundity in outcrossing species might at times be advantageous.     

LOCALIZED ADAPTATION OF CLONES OF THE SEA ANEMONE ACTINIA TENEBROSA

Adult Actinia tenebrosa were reciprocally transplanted within and between colonies separated by 2 or 4 km. These experiments revealed powerful genetic and environmental effects on adult size and asexual fecundity and demonstrate that clones may be highly locally adapted. Local adults displayed significantly greater asexual fecundity than do adults transplanted from other colonies, although all groups showed similar survivorships. This provides the first demonstration of fine scale adaptation, in either terrestrial or marine environments, for a clonal species with large, panmictic sexually breeding populations. These findings, together with the results of earlier studies of this species, strongly support the assumptions and predictions of the Strawberry-Coral Model (Williams, 1975).     

Colonisation and competition dynamics can explain incomplete sterilisation parasitism in ant–plant symbioses

Sterilisation of parasites prevents host reproduction, thereby diverting host resources to their own benefit. Previous theory predicts that parasites should evolve maximum virulence, yet hosts are often incompletely sterilised. Whereas prior attempts to resolve this paradox have sought evolutionary explanations, we present theory and experiments showing that incomplete sterilisation can arise from ecologically driven fluctuations in parasite load. The African ant–plant Acacia drepanolobium reproduced more when occupied by small colonies of the sterilising symbiont Crematogaster nigriceps. In nature, small colonies result from interference competition between ant colonies; these territorial conflicts thus provide intermittent windows of opportunity for host reproduction. Our mean‐field model shows that numerical insufficiency of parasites can produce partial sterilisation of host populations, creating the appearance of reduced virulence even if ants have evolved to sterilise completely. This general framework helps explain both the apparent ubiquity of partial sterilisation parasitism and the ability of these symbiotic associations to persist.     

Variation in patriline reproductive success during queen production in orphaned colonies of the thelytokous ant Cataglyphis cursor

In genetically diverse insect societies (polygynous or polyandrous queens), the production of new queens can set the ground for competition among lineages. This competition can be very intense when workers can reproduce using thelytoky as worker lineages that manage to produce new queens gain a huge benefit. Selection at the individual level might then lead to the evolution of cheating genotypes, i.e. genotypes that reproduce more than their fair share. We studied the variation in reproductive success among worker patrilines in the thelytokous and highly polyandrous ant Cataglyphis cursor. Workers produce new queens by thelytoky in orphaned colonies. The reproductive success of each patriline was assessed in 13 orphaned colonies using genetic analysis of 433 workers and 326 worker-produced queens. Our results show that patrilines contributed unequally to queen production in half of the colonies, and the success of patrilines was function of their frequencies in workers. However, over all colonies, we observed a significant difference in the distribution of patrilines between workers and worker-produced queens, and this difference was significant in three of 13 colonies. In addition, six colonies contained a low percentage of foreign workers (drifters), and in one colony, they produced a disproportionably high number of queens. Hence, we found some evidence for the occurrence of rare cheating genotypes. Nevertheless, cheating appears to be less pronounced than in the Cape Honey bee, a species with a similar reproductive system. We argue that worker reproduction by parthenogenesis might not be common in natural populations of C. cursor.     

Arms races between social parasites and their hosts: geographic patterns of manipulation and resistance

The evolutionary interactions between permanently social parasiticspecies and their hosts are of special interest because socialparasites are not only closely dependent on, but are also closelyrelated to, their hosts. The small European slavemaker Harpagoxenussublaevis has evolved several characters that help manipulateits host. In this study we investigated adaptations of thissocial parasite to its local hosts and the geographic patternof host resistance in two main host species from three differentpopulations. In behavioral experiments, we examined whetherhost colonies from three geographically distant Leptothoraxacervorum populations varied in their ability to defend thenest against social parasites. Naive colonies from the unparasitizedEnglish population killed attacking slavemakers more often thandid host colonies from two parasitized populations. We alsofound strong interpopulation variation in the ability of theslavemaker to manipulate host behavior. H. sublaevis uses theDufour gland secretion to induce intracolonial fights and, ingeneral, this "propaganda" substance was most effective againstlocal hosts. Our results suggest that the social parasite isleading the arms race in this aspect. Similar experiments uncovereddifferences between two populations of the second host speciesL. muscorum and could demonstrate that nest defense in bothhost species is similarly efficient. In L. acervorum, monogynouscolonies were more successful in nest defense, whereas socialstructure had no impact in L. muscorum. Colony size did notaffect the efficacy of nest defense in either host species.The caste of the slavemaker had a strong influence on the successof an attack.     

A predictive relationship between population and genetic sex ratios in clonal species

Sexual reproduction depends on mate availability that is reflected by local sex ratios. In species where both sexes can clonally expand, the population sex ratio describes the proportion of males, including clonally derived individuals (ramets) in addition to sexually produced individuals (genets). In contrast to population sex ratio that accounts for the overall abundance of the sexes, the genetic sex ratio reflects the relative abundance of genetically unique mates, which is critical in predicting effective population size but is difficult to estimate in the field. While an intuitive positive relationship between population (ramet) sex ratio and genetic (genet) sex ratio is expected, an explicit relationship is unknown. In this study, we determined a mathematical expression in the form of a hyperbola that encompasses a linear to a nonlinear positive relationship between ramet and genet sex ratios. As expected when both sexes clonally have equal number of ramets per genet both sex ratios are identical, and thus ramet sex ratio becomes a linear function of genet sex ratio. Conversely, if sex differences in ramet number occur, this mathematical relationship becomes nonlinear and a discrepancy between the sex ratios amplifies from extreme sex ratios values towards intermediate values. We evaluated our predictions with empirical data that simultaneously quantified ramet and genet sex ratios in populations of several species. We found that the data support the predicted positive nonlinear relationship, indicating sex differences in ramet number across populations. However, some data may also fit the null model, which suggests that sex differences in ramet number were not extensive, or the number of populations was too small to capture the curvature of the nonlinear relationship. Data with lack of fit suggest the presence of factors capable of weakening the positive relationship between the sex ratios. Advantages of this model include predicting genet sex ratio using population sex ratios given known sex differences in ramet number, and detecting sex differences in ramet number among populations.     

Field data do not support a textbook example of convergent character displacement

Experimental evidence supporting convergent character displacement is rare; only one example exists and it is in the form of orientation and territory competition experiments performed in the laboratory. However, outcomes of laboratory experiments involving behaviour or competition can be artefacts of unnatural conditions and, therefore, the results of the previous experiments supporting convergent character displacement are equivocal. In this study, we re-examine the evolution of melanic nuptial coloration in male three-spined stickleback (Gasterosteus aculeatus) inhabiting the Chehalis River drainage in Washington State. This novel nuptial coloration has been thought to have evolved in response to competition for nesting territories with the co-distributed Olympic mudminnow (Norzumbra hubbsi), which is also melanic and breeds at the same time. I found that melanic stickleback males did not have an advantage over their red counterparts from typical populations when competing for nesting territories with Olympic mudminnows. Additionally competitive interactions between sticklebacks and mudminnows were rare in both cage experiments and naturally breeding sticklebacks. Finally, melanic coloration in the Chehalis populations did not develop until males were parental, well after the hypothesized territory establishment period. These results refute the only experimental support for convergent character displacement and emphasize the importance of conducting behavioural experiments and observations under natural conditions.     

Two alternate mechanisms contribute to the persistence of interdependent lineages in Pogonomyrmex harvester ants

Some populations of Pogonomyrmex harvester ants comprise pairs of highly differentiated lineages with queens mating at random with several males of their own and of the alternate lineage. These queens produce two types of diploid offspring, those fertilized by males of the queens' lineage which develop into new queens and those fertilized by males of the other lineage which mostly develop into functionally sterile workers. This unusual mode of genetic caste determination has been found in 26 populations and a total of four lineage pairs (F(1)-F(2), G(1)-G(2), H(1)-H(2) and J(1)-J(2)) have been described in these populations. Despite the fact that a few interlineage queens are produced, previous studies revealed that there is a complete lack of genetic introgression between lineages. Here we quantify the proportion of interlineage queens produced in each of the four lineage pairs and determine the fate of these queens. In the F(1)-F(2), G(1)-G(2) and H(1)-H(2) lineage pairs, interlineage queens were produced by a minority of colonies. These colonies exclusively produced interlineage queens and workers, suggesting that interlineage eggs can develop into queens in these three pairs of lineages in the absence of competition with pure-lineage brood. An analysis of three key stages of the colony life cycle revealed that colonies headed by interlineage queens failed to grow sufficiently to produce reproductive individuals. In laboratory comparisons, interlineage queens produced fewer viable eggs, with the effect that they raised fewer workers and lost more weight per worker produced than pure-lineage queens. In the J(1)-J(2) lineage pair, we did not find a single interlineage queen, raising the possibility that interlineage eggs have completely lost the ability to develop into queens in this lineage pair. Hence, two distinct mechanisms seem to account for the complete lack of between-lineage gene flow in the F(1)-F(2), G(1)-G(2), H(1)-H(2) and J(1)-J(2) lineage pairs.     

Apparent competition through facilitation between Melanoides tuberculata and Biomphalaria glabrata and the control of schistosomiasis

Interactions between two species that result in reduced growth rates for both and extinction of one of the species are generally considered cases of asymmetric interspecific competition. Exploitative or interference competition is the usual mechanism invoked. Here we describe another mechanism producing the same result, named apparent competition through facilitation (ACF), observed between Melanoides tuberculata and Biomphalaria glabrata populations. The superior competitor actually gives some benefit to the other species, whose population becomes unstable with progressively increasing oscillations, leading to extinction. A model of ACF using difference equations suggests initial dynamics distinct from traditional interspecific competition. The dynamics of two freshwater snails in the field and in laboratory experiments suggest ACF, and these relations should be considered in studies of schistosomiasis control. ACF could occur in natural populations, but might have gone undetected because the final result is similar to traditional interspecific competition.